1. Field of the invention
The present invention relates to a hybrid notch filter arrangement which employs a unique combination of active and passive elements, but more specifically the present invention relates to a "semi-active" notch filter wherein the low frequency phase shift and the value of the notch frequency thereof can be controlled independently.
2. Description of the Prior Art
In the prior art, a need has been established, in certain phase-locked-loop applications (in modeling the temperature and frequency dependence of the Josephson effects for example), for a low-pass filter or notch filter having minimal phase shift at low frequencies and good rejection characteristics at high frequencies. As is well known, the foregoing two properties are inter-related. Accordingly, in general, optimization of one property can only be accomplished at the expense of the other property. Hence, there is a need in the prior art for a notch filter which allows independent control of the low-frequency phase shift and the high frequency attenuation, i.e., the notch frequency.
Prior art techniques of stability control, in phase-locked-loops, include the use of low-pass filters (active and passive) and notch filters (active and passive). The major disadvantage of these filters is their excessive phase shift at low frequencies. For example, the well known twin-T filter has a low frequency phase shift 2 to 3 times greater than the acceptable low frequency phase shift in a phase-locked-loop for simulating the Josephson effects. Consequently, there is a need in the prior art to configure a filter having a frequency response with many of the characteristics of the twin-T, but yet having a low frequency phase shift of substantially zero.
In addition, it has been found that active low-pass and notch filters, built using conventional operational amplifiers, do not function properly at the frequencies of interest for Josephson effects simulation (0-300 kHz) because the gain of the operational amplifiers is so small at those frequencies. The design of most of these active filters is based on the assumption that the gain of the operational amplifier used therewith is approximately infinite, while in actual practice it is finite. It has also been found that, in general, the input impedance of passive filters is generally not as high as desired; nor is their output impedance as low as generally desired. Thus, there is a need in the prior art to configure a notch filter so as to provide a "semi-active" notch filter wherein the notch frequency is controlled primarily by the passive filters and the low frequency phase shift is controlled primarily by the properties of the operational amplifiers used.
As additional background material, a Josephson effects simulator in which the present invention can be used is disclosed, inter alia, in a Navy Technical Disclosure Bulletin Article to Jablonski, entitled "Improved Josephson Analogue System For Modeling Superconductive Tunneling", Vol. 7, No. 4, June 1982, pgs. 82-85.
The prior art include some advances in low-pass and notch filter design, but as far as can be determined, no prior art filter incorporates all of the features and advantages of the present invention.